Over the past two years we have studied the emission of fluorescence from biological molecules under excitation by fast electrons in the scanning electron microscope. Such fluorescence is characteristic of the molecular species and so can serve for molecular identification at high space resolution. Most fluorescing groups (fluorophors) are rapidly destroyed by the beam, giving up far too few photons to be detected on an individual basis. Nucleic acid is an exception though at present we still require a cube of nucleic acid several hundred A on a side for detection with identification. We have recently constructed a fluorescent antibody which is long-lived under the beam; it should permit immuno-fluorescence EM at a resolution of a few hundred A or better. Three other high-resolution probes have been found. One of these may have infinite lifetime. If so, it will serve to test the predicted ultimate resolution of 15-20 A. The technique will be applied to two general classes of problem which might be called "resolution of biological structures into identified components" and "study of biological function by localization of key molecular species". An example of the first would be resolution of the fine structure of chromatin into a part due to DNA and a part due to a particular histone (identified by fluorescent antibody). An example of the second would be a connection between actin-containing cables in the cytoplasm and a component of the cell membrane which depends on viral transformation.